1. Field of the Invention
The present invention relates to a wavelength converting element and a method of manufacture thereof, and in particular, to a wavelength converting element which converts a fundamental wave to a second harmonic, and to a method of manufacturing the wavelength converting element.
2. Description of the Related Art
Conventionally, in a wavelength converting element, a waveguide is formed by a Ti thermal diffusion method or a proton exchange method. In these methods, because Ti or protons are diffused toward the interior from the surface of an optical crystal substrate, a flat waveguide which is formed from region having a high refractive index is formed in a vicinity of the surface. Accordingly, the shape of laser light which is guided in the waveguide is flat, and problems arise in the joining of the wavelength converting element with a lens or an optical fiber or the like. Further, because the second harmonic is confined at the high refractive index side with respect to the fundamental wave, there is the problem that there is little overlapping of the fundamental wave and the second harmonic, and the wavelength converting efficiency is poor.
In order to overcome these problems, a light waveguide in which a clad layer of a high refractive index is formed at a substrate surface is proposed in Japanese Patent Application Laid-Open (JP-A) No. 9-281536. Further, in JP-A No. 11-72810, a light waveguide, in which a high refractive index layer is formed in a vicinity of the surface of a substrate by carrying out proton exchange twice, is proposed. In these light waveguides, due to the high refractive index layer being formed at the surface of the substrate, the peak of the fundamental wave is moved to a vicinity of the substrate, and the overlapping with the second harmonic is made to be great, and the converting efficiency is increased. However, these structures are no different in that the vicinity of the substrate is a flat waveguide of a high refractive index. Because the peaks of the fundamental wave and the second harmonic do not coincide, the improvement in the wavelength converting efficiency is limited, and control of the beam shape is difficult.
On the other hand, Japanese Patent No. 2765112 proposes forming a light wavelength converting element having improved wavelength converting efficiency in which two substrates, in which inverted domains and waveguides are formed, are set to oppose one another, and their positions are made to match one another by an aligner, and the substrates are laminated together. However, although the converting efficiency of this light wavelength converting element is high, the two substrates must be fabricated separately by substantially the same process, and there is the problem that the manufacturing cost is high because the two substrates cannot be fabricated simultaneously. Further, at the time of laminating the two substrates together, because the light waveguides and the inverted domains themselves are transparent, it is difficult to visually confirm the respective positions of the substrates, and there is a problem that the light converting efficiency of the element deteriorates due to errors in alignment which arise due to the difficulty of visual confirmation.
The present invention has been proposed in order to overcome the above-described drawbacks, and an object of the present invention is to provide a wavelength converting element having a beam shape suitable for joining with a lens or an optical fiber, and a method of manufacturing the wavelength converting element in which a deterioration in yield is suppressed and the manufacturing costs are kept low.
A first aspect of the present invention is a wavelength converting element comprising: a lower substrate having a waveguide which is formed by proton exchange, and inverted domains which are formed periodically with respect to a light advancing direction of the waveguide; and an upper substrate which is laminated to the lower substrate so as to oppose the waveguide.
It is not necessary for a waveguide to be formed in the upper substrate in the same way as the lower substrate, and it suffices to, for example, merely wash the upper substrate. Further, due to the upper substrate being laminated to the lower substrate, the waveguide is positioned in a vicinity of the boundary surface between the upper substrate and the lower substrate. Accordingly, in this wavelength converting element provided with a waveguide, the waveguide is formed at the interior of the substrate. Thus, a deterioration in yield, which is due to chipping of edges at the time of polishing or handling, which is normally a problem in waveguide devices which are fabricated by diffusion from the surface, is suppressed. Further, a light loss due to dust or the like adhering to the waveguide can be prevented.
It is preferable that a thickness of the upper substrate is thinner than a thickness of the lower substrate. In this way, at the time of lamination, the upper substrate is deformed, and deformation of the lower substrate can be suppressed. As a result, changing of the phase matching condition due to distortion of the lower substrate can be suppressed.
Further, the waveguide may be formed by proton exchange in the lower substrate at which inverted domains are periodically formed, and the upper substrate may be laminated to the lower substrate so as to oppose the waveguide.
Without a special processing being carried out on the upper substrate, the upper substrate is laminated to the lower substrate so as to oppose the waveguide which is formed at the lower substrate. Because highly precise alignment is not required at this time, a decrease in the yield due to errors in alignment can be avoided.
Further, it is preferable that a heat treatment is carried out in a state in which the upper substrate and the lower substrate are laminated together, and protons, which are exchanged at the lower substrate, diffuse into the upper substrate, and simultaneously, the upper substrate and the lower substrate are joined. Due to the protons diffusing while the heat treatment is carried out, the upper substrate and the lower substrate can be strongly laminated together. Moreover, the waveguide can be formed in a vicinity of the boundary surface between the upper substrate and the lower substrate.
It is preferable that, before the lower substrate and the upper substrate are laminated together, a region between the lower substrate and the upper substrate is deaerated. By carrying out deaeration in this way, at the time of laminating the lower substrate and the upper substrate together, a generation of air pockets between the substrates can be prevented. Further, if a concave portion is formed in the surface of the substrate, the interior thereof is set in a state of reduced pressure, and because the lower substrate and the upper substrate are pressed by atmospheric pressure, both substrates are fit even more tightly together, and are difficult to peel apart from one another. Moreover, in a case in which air pockets caused by particles which are caught between the upper and lower substrates are formed, the air may expand during the heat treatment, and the upper and lower substrates may peel apart from each other over a wide range. However, by forming a concave portion, the expanded air can be absorbed at the concave portion. Therefore, peeling of the substrates from each other can be suppressed, and the yield improves.
The wavelength converting element relating to the present invention has a lower substrate having a waveguide which is formed by proton exchange, and inverted domains which are formed periodically with respect to the light advancing direction of the waveguide; and an upper substrate which is laminated to the lower substrate so as to oppose the waveguide. Thus, joining with a lens or a fiber is easy, and it is possible to obtain a beam shape of a high wavelength converting efficiency.
In the method of manufacturing a wavelength converting element relating to the present invention, an upper substrate is laminated to a lower substrate having a waveguide which is formed by proton exchange and inverted domains which are formed periodically with respect to the light advancing direction of the waveguide, such that the upper substrate opposes the waveguide. Thereafter, by carrying out a heat treatment, the protons diffuse into the upper and lower substrates, and simultaneously, the upper and lower substrates are joined together. In this way, abeam shape, which facilitates joining with a lens or fiber, and a waveguide wavelength converting element having high wavelength converting efficiency can be manufactured easily and at a high yield.